This invention relates to a covering device and a covering material comprising high or ultra high molecular weight polyethylene (UHMWPE). The invention is also directed to cord-like devices such as ropes, cords, cables, conduits, strands of fibers and tapes, etc. comprising the covering device or the covering material mentioned above with superior abrasion resistance, reduced weight and diameter.
New ropes are continually being developed to meet the needs of specialized applications. Through rope engineering, it is possible to design a rope with specific performance characteristics. These characteristics are met by changing materials and construction methods. Decisions as to what factors are important must be made.
In a practical sense, advantages and disadvantages must be traded off and compromises must be made to design the best rope for a given application.
Some applications require: Strong low-elongation, light weight and high abrasion resistance ropes for rescue; Shock-absorbing ropes for the rock climber; Floating ropes for marine uses and river rescue; Low-elongation, shock absorbing ropes for caving and arborists; Soft responsive rappel ropes; Colored rope for multi-rope management challenges; and Superior strength, non stretching buoyant ropes for marine applications.
In general, ropes are constructed with multifilament fibers in three strands twisted together or in various braided forms. The various multifilament fibers used in rope manufacturing are predominantly synthetic and made of polymers such as polyamides (e.g. Nylon), polyesters (e.g. Dacron®, polypropylene, polyethylene (e.g. Dyneema® and Spectra®), aromatic polyamides (e.g. Kevlar®, Twaron®) and aromatic co-polyesters (e.g. Vectran®).
Depending on the intended purpose of the rope, different polymer fibers are used; polyamides for their stretching and shock absorbing capability, polyesters for their UV and abrasion resistance, polypropylene for its lightness and low cost, polyethylene and aromatic polyamides for their high strength and low extension.
However, these commonly used rope fibers also have certain disadvantages that are associated with their particular chemical structures. Polyamides and to a lesser extent polyesters absorb water, polyamides and polypropylene are susceptible to UV degradation, and aromatic polyamides are susceptible to fatigue bending.
Also, whereas some of the rope fibers have higher abrasion resistance than others, e.g. polyesters over polyamides, ultra high molecular weight polyethylene over aromatic polyamides, all rope fibers are made of bundles of multifilament fibers that is structures that are produced to have a high degree of chain orientation and extension and be strong in their axis direction. As a result they are very weak in their lateral directions perpendicular to their axis. For example, in high strength polyethylene fibers, the molecular chains are held by weak van der Waals forces in the lateral directions. Because of the very weak lateral bonding between the filaments of multifilament fibers, when ropes made of multifilament fibers are rubbed against other materials encountered during their use, for example, rocks, stones, cement, and salt crystals, their filaments shred and break down into weaker microfilaments, a process that leads to the weakening and destruction of the rope. The greater the degree of shredding and break down of filaments and fibers the greater the destruction of the rope.
In order to obtain the benefit of individual polymer properties and manage the cost of the ropes particularly those containing the costly high strength fibers of ultra high molecular weight polyethylene and aromatic polymers, manufacturers combine different polymer fibers for the construction of hybrid ropes, for example a high strength ultra high molecular weight polyethylene or aromatic polyamide as a core with a less expensive polyamide, a UV resistant polyester or a light weight polypropylene as a braided outside layer to protect the high strength core component.
This outside layer, or protective cover, is used to protect the high strength performance core component for reasonable lengths of time, and is used in proportions of 40-60% of the total weight of the rope for example. It contributes a significant amount to the weight of the rope for the role of protection as well as to the overall volume of the rope, as the diameter is typically substantially larger than that of the core component. This protective layer can make the rope heavier, bulkier and more difficult to handle.
Prior art shows various other efforts to improve the abrasion resistance and hence the service life of the ropes. For example, U.S. Pat. No. 4,534,163 (Issued to Schuerch on Aug. 13, 1985) shows the use of a urethane coating to impregnate a polyethylene terephthalate fabric in the form of strips wound around the core fibers and curing it to form a protective jacket for abrasion. Polyurethanes are thermoplastic elastomers and may exhibit better abrasion resistance over other rubbery materials, however they fracture into debris between the fibers and can contribute in the destruction of the fibers (Sloan, F., S. Bull, and R. Longerich. “Design Modifications to Increase Fatigue Life of Fiber Ropes.” Oceans. 1. (2005): 829-835). Such failure mechanism shows that polyurethanes provide a limited benefit as a coating and a binder. Moreover, the addition of polyurethane to form a protective jacket results in additional weight and larger volume, i.e. the rope becomes heavier and larger in diameter. PCT Patent Application no. WO 98/50621 (to Moraes Del Vecchio et al., published Nov. 12, 1998) reports (page 2, line 25-page 3, line 12) that the application of polyurethane coatings has been tested and found to have also other disadvantages, e.g. detachment, concentration of stresses and strength reduction and manufacturing problems.
WO 98/50621 shows another approach to improve the internal abrasion of the core fibers by using underneath the outside braided protective layer a polyethylene strip placed helically on the core fibers in order to prevent seabed debris that pass through the outside protective layer from reaching the core fibers and cause their destruction by abrasion. Such an arrangement does not protect the outside braiding layer of e.g. nylon or Dacron which rubs against the seabed nor the additional layer of the polyethylene strip which relies on the outside layer for its protection. In other words, it is expected that the polyethylene under-layer to have an inferior abrasion resistance to the outside protective layer.
Therefore, there is a need in the industry for a covering device or a covering material that responds to at least one drawback of what is known in the field. More particularly, there is a strong need for a new covering material or coating allowing the manufacture of a new generation of ropes that can provide high mechanical performance and other useful attributes such as floating capabilities with enhanced abrasion resistance and hence longer service lifetime in smaller diameters and lower weights than currently used traditional ropes.